Various faults could occur in an optical repeatered undersea system and if the fault needs to be repaired then it is important to be able to determine accurately the location of the fault so that the repair ship can get close to the fault when it recovers the cable. Otherwise if the fault is found to be some distance from the point of cable recovery, much time and effort can be wasted.
Faults fall basically into two categories. An optical fault while the system remains powered and an electrical fault in the power feed system. Both optical and electrical faults can occur together.
A known technique for optical fault location relies upon a loopback arrangement in the repeaters whereby on command from a terminal at an end of the system, each repeater in turn has the go signal fibre path looped back to the return fibre path. When the faulty repeater or cable section is reached then either the looped back signal fails or is corrupted so that the section or repeater can be identified.
Another technique relies upon the received light level coming into a regenerator and locating the regenerator which receives no or diminished light, to once again determine the leg or repeater which is faulty.
As the repeater spacing is becoming larger with improved fibre properties, currently around 60 or 70 kilometres, then this system is rather crude because the fault can lie anywhere along a 60 or 70 kilometre length of cable. The exact location is determined by lifting the cable at some point along the length containing the fault, cutting the cable and using for example a back scatter technique from the cut end to determine where from the cut the fault lies. If it is far from the cut then the cut is repaired by inserting a short fresh cable length across the cut end, relaying the recovered cable with its repaired cut, and then proceeding to the precise location of the fault as determined by the back scatter measurement, and subsequently making a repair by recovering and repairing the cable.
A paper entitled "An automatic optical fibre break location scheme for duplex and diplex transmission systems" in Br. Telecom Technology Journal Vol 6, No 1 January 1988 at page 54, provides a solution to locating a fibre break. This technique relies on observing and comparing the arrival time of the signal represented by loss of data or loss of light, in the broken fibre. The authors report that it is fairly accurate in a duplex system where the fibre operates at different wavelengths in opposite directions but no so accurate for a diplex system where operation at different wavelengths is in the same direction. This technique is not suitable however for current submarine systems, which are simplex systems.
When an electrical fault occurs caused e.g. by a trawler or anchor mauling the cable, without breaking it, then if the fault causes a short circuit he location of the fault can be made using the fact that at the fault the electrical conductor carrying the power is shorted to earth (the sea). The system will be powered from both ends, positive at one end and negative at the other. By adjusting the terminal voltages so that no current is lost at the fault i.e. the voltage gradient reaches zero at the fault, then the location of the fault can be found since the distance of the fault from either terminal will be in proportion to the voltage at that terminal required to produce zero current at the fault. Other methods involving resistance and voltage measurement are possible also.
If the electrical fault results in an open circuit then a capacitive technique can be used based on a knowledge of the capacitance per unit length of the cable.
None of the above techniques however is suitable if an intermittent electrical fault occurs.
It is an object of the present invention to devise a technique to locate an intermittent electrical fault in a simple manner.